The invention relates to a tundish having a refractory lining for producing and transferring high-purity metal melt from a casting ladle into the permanent mold of a continuous casting installation, and to a process for producing a high-purity metal strand using a continuous casting installation.
During the continuous casting of metal strands, in particular during the continuous casting of steel, a tundish is usually fitted between the casting ladle and the continuous casting permanent mold, in order to compensate for fluctuations in the supply of melt and in the rate at which the metal strand is drawn off from the continuous-casting installation. Especially in the case of sequence casting, it is necessary to store a sufficient quantity of metal melt in the tundish to span the time required to change the ladle.
The melt is usually transferred from the tundish to the permanent mold of a continuous casting installation through an outflow opening in the tundish base, which is assigned a controllable closure member, such as a slide or a stopper, and also through a submerged casting pipe or a casting nozzle. The permanent mold may be designed in a very wide range of ways, for example it may be an oscillating tube or plate mold, a mold formed by a single casting roll or by two interacting casting rolls and side plates, or a mold formed by revolving belts or tracks.
In the case of multi-strand casting installations, this tundish is designed as a distributor vessel and, via a plurality of melt outlets, supplies a plurality of continuous casting permanent molds arranged next to one another. V-shaped distributor vessels are known for two-strand casting installations.
Furthermore, the tundish is usually used to calm the metal melt which flows in from the casting ladle and is supposed to allow slag particles and other nonmetallic inclusions to be separated out during the residence time of the metal melt in the tundish. To ensure that this is achieved to a sufficient extent, the flow properties of the metal melt are usually deliberately influenced by flow-guiding internal fittings in the tundish. Trough-like tundishes formed in this way are already known, for example from EP-B 804 306 and EP-A 376 523.
If the flow and temperature characteristics in a trough-shaped tundish, as has been used for decades in conventional steelmaking processes and continuous casting installations, are considered, liquid steel is introduced from the casting ladle, via a shroud, into a manifold vessel or tundish. The induced steel jet flows toward the tundish base, where it strikes the flat base of the tundish or a flow-diversion device, which diverts the jet of liquid toward the bath level surface and extracts kinetic energy through dissipation. In the inlet region, the flow generally returns to the bath level surface, migrates along the latter and is submerged again along the narrow back wall and along the side walls of the trough-shaped tundish. As a result, depending on the shape of the tundish, substantially two oppositely rotating recirculation rolls (upward flow in the longitudinal-center section), which migrate in the direction of the outlet opening, are induced. The jet temperature decreases in the direction of the outlet opening as a result of heat losses via the side walls and the bath level surface, with the temperature loss between the feed location and outlet location being dependent on the throughput.
The foreign substances in the metal melt, which are to be separated out as efficiently as possible, originate firstly from the steelmaking process, and are flushed out of the casting ladle into the tundish when the metal melt is transferred. Secondly, foreign substances are also introduced into the metal melt in the tundish itself. These foreign substances originate from the refractory lining material of the tundish and/or from the liquid steel covering slag which is generally used, and are abraded and suspended firstly through mechanical erosion as a result of wall shear stresses or through chemical erosion resulting from reoxidation processes. Furthermore, inclusions of slag are formed through resuspension on account of high bath level velocities and increased surface turbulence.
Therefore, it is an object of the present invention to avoid the drawbacks which have been outlined and to propose a tundish and a process for producing a metal strand in which the reintroduction of particles into the metal melt within the tundish is minimized and overall the maximum possible separation rate for all the inclusions which are present in the metal melt is achieved, so that a melt which is as pure as possible is fed to the permanent mold.